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ISTEW    HAVE]^,     COFTsT. 


BULLETIN    No.    112, 

JUNE,    1892. 


ON  THE   GUNNING-KJELDAIIL   METHOD    AND   A 

MODIFICATION   APPLICABLE   IN   THE 

PRESENCE   OF  NITRATES. 

By  A.  L.  WiNTON,  Jr. 

The  Giinnbig  MetJiod  [in  absence  of  nitrates). 

The  modification  of  the  Kjeldahl  method,  proposed  by  Gunning 
in  1889,  has  since  been  indorsed  by  a  number  of  well-known 
chemists. 

The  Gunning  method  is  very  simple — the  only  reagents  used 
previous  to  the  distillation  with  caustic  soda  being  sulphuric  acid 
and  potassium  sulphate,  the  latter  taking  the  place  of  the  three 
reagents — mercury,  (oxide  or  metallic),  potassium  permanganate 
and  potassium  sulphide— ordinarily  used. 

The  mixture  of  sulphuric  acid  and  potassium  sulphate  boils  at  a 
high  temperature,  and,  as  a  rule,  oxidizes  the  organic  substance 
more  rapidly  than  the  ordinary  mixture  of  sulphuric  acid  and 
oxide  of  mercury. 

The  results  obtained  by  Gunning,*  Atterberg,f  Van  SlykeJ  and 
others,  including  the  writer,  on  fodders,  dairy  products,  fertili- 
zers free  from  nitrates,  and  various  organic  substances,  show  that 
this  method  is  fully  as  accurate  as  the  ordinary  Kjeldahl  method, 
and,  in  some  cases,  gives  better  results.  It  is  a  well  known  fact 
that  the  nitrogen  of  many  alkaloids,  azo-compounds  and  various 
bodies  of  the  aromatic  series  cannot  be  determined  by  the  usual 
Kjeldahl  method,  but  Gunning  and  Atterberg  have  obtained  good 
results  on  morphine,  quinine,  indigo  and  aniline  oxalate  by  the 
Gunning  modification. 

*  Fres.  Zeit.,  28,  188. 

f  Chem.  Zeitnng,  14,  509. 

X  U.  S.  Dept.  Agr.  Div.  Chem.  Bull.,  31,  142. 


no.  ha. 


2 

The  chief  disadvantage  of  the  method  is  the  frothing  wliich 
occurs  during  tl)e  first  part  of  tlie  heating.  This  fi-othing,  Ijow- 
evcr,  causes  but  little  trouble  if  a  flask  of  at  least  500  c.c.  ca- 
pacity (which  can  afterwards  be  used  for  the  distillation),  is  em- 
ployed and  care  be  taken  in  adjusting  the  heat. 

The  Gunning  mixture  is,  at  ordinary  temperatures,  half-solid, 
and  must  be  heated  before  it  can  be  measured  for  use.  It  is 
therefore  best  to  add  18  grams  of  potassium  sulphate  and  20  c.c. 
of  sulphuric  acid,  separately  and  in  the  order  named,  to  the  flask 
containing  the  substance,  shaking  a  few  times  before  heating. 
The  coarsely  powdered  sulphate  may  be  conveniently  measured 
out  in  a  cai'tridge-shell  adjusted  to  hold  18  grams  and  provided 
with  a  wire  handle.  The  acid  may  be  added  from  a  graduated 
cylinder  or  a  burette  of  wide  calibre  connected  by  a  siplion  with 
the  acid  reservoir.* 

After  the  digestion  is  completed  water  should  be  added  before 
the  mixture  becomes  too  cold,  otherwise  it  is  difficult  to  obtain  a 
solution  of  the  solid  mass. 

The  writer  has  found  that  the  fertilizer-chemical  known  in  the 
trade  as  "  High  Grade  Sulphate  of  Potash,"  and  costing  but  a  few 
cents  a  pound,  answers  every  purpose,  and  is,  in  fact,  better  than 
the  white  sulphate  of  the  apothecary,  being  usually  free  from 
moisture  and  chlorides,  the  presence  of  which  tends  to  increase  the 
frothing. 

Following  is  a  comparison  of  results,  obtained  by  the  methods 
named,  on  those  forms  of  organic  nitrogenous  matter  which  are 
most  commonly  used  in  mixed  fertilizers,  and  on  four  commercial 
fertilizers  free  from  nitrates. 

Kjelclahl  Melbod.  Gunning-KJeldahl  Wetiiod 

Cotton  Seed  Meal 7.06  7.11 

"        "    7.10  7.13 

"        "     7.10  7.1.5 

"         "        "    6.95  6.97 

Castor  Pomace 5.56  5.59 

Tankage 5.03  5.07 

"    5.72  5.82 

Bone 4.03  4.02 

"  3.92  3.90 

Dry  Ground  Fish .- - 8.69  8.77 

"                 "     8.14  8.06 

Peruvian  Guano 2.88  2.95 

Mixed  Fertilizerf 3.45  3.45 

X 3.00  3.00 

"               "         § 3.80  3.76 

"               "             1.82  1.81 

*  Report  Conn.  Agl.  Expt.  Station,  1889,  192. 

f  Contains  1.89  per  cent.  Nitrogen  in  Ammonia  salts. 

i         '•        1.76"    " 

8        "       1.77       " 


Tim  Modified  Method  appUcahle  in  the  presence  of  Nitrates. 

In  laboratories  where  fertilizer  analyses  are  being  made,  the 
adoption  of  the  Gunning  method  wonld  tend  to  complicate  rather 
than  to  simplify  matters  so  long  as  in  the  presence  of  nitrates 
nitrogen  is  determined  by  another  method. 

It  is  impracticable  to  add  salicylic  acid  to  Gunning's  reagent 
as  such  a  mixture  could  only  be  used  while  hot  and  it  is  impor- 
tant that  the  temperature  be  kej^t  as  low  as  possible  during  the 
early  stages  of  the  digestion.  It  occurred  to  the  writer,  however, 
that  Scovell's  mixture  of  sulphuric  and  salicylic  acids  might  be 
used  in  the  ordinary  way,  potassium  sulphate  being  added  later 
in  the  process  in  place  of  mercury.  After  a  number  of  unsuc- 
cessful trials — unsuccessful  because  too  much  potassium  sulphate 
was  used  and  the  boiling  point  of  the  liquid  became  so  high  that 
oxides  of  nitrogen  were  driven  off — the  following  plan  was  settled 
iipon. 

The  material,  .5  to  1.0  gram,  is  digested  with  80  c.  c.  of  Sco- 
vell's salicylic  acid  mixture  (30  c.  c.  of  sulphuric  acid  and  2  gms. 
salicylic  acid)  in  a  flask  of  600  c.  c.  capacity,  for  two  hours  with 
frequent  shaking.  Two  grams  of  zinc  dust  are  then  slowly 
added,  with  continual  shaking,  and  tlie  flask  heated,  at  first  gently, 
until  after  a  few  minutes  boiling,  dense  fumes  are  no  longer  given 
off.  Thus  far  the  process  is  the  same  as  in  the  Scovell-Kjeldahl 
method  (the  oflicial  method  of  the  Association  of  Ofiicial  Agri- 
cultural Chemists)  except,  that  the  digestion  is  continued  for  two 
hours,  which,  in  some  cases,  the  writer  has  found  absolutely 
necessary  in  order  to  secure  comjjlete  solution  of  the  nitrate. 
Ten  to  twelve  grams  of  potassium  sulphate  are  next  added  and 
the  boiling  continued  for  a  little  time  after  the  solution  is  color- 
less or,  if  iron  is  present,  has  a  lighc  straw  color  which  remains 
unchanged.  On  cooling,  as  the  mixture  begins  to  solidify,  water 
is  added,  at  first  slowly  with  shaking,  and  the  distillation  with 
caustic  soda  is  cari-ied  on  in  the  usual  manner. 

The  trials  thus  far  made  with  this  method  on  nitrates  and  fer- 
tilizers containing  nitrates,  have  proved  entirely  satisfactory. 

In  the  following  table  are  given  the  results  obtained  by  the 
official  method  and  the  method  here  described,  together  with  the 
percentage  of  nitrogen  in  nitrates  as  determined  by  the  Schulze- 
Tiemaun  method. 

The  average  of  the  25  determinations  by  the  Scovell-Jodlbauer 
method  is  4.64  per  cent.,  by  the  proposed  method  4.65.  The 
greatest  discrepancy  in  any  instance  is  one-tenth  per  cent., 
the  average  discrepancy  five  onehundredths  per  cent.  In  ten 
cases  the  proposed  method  gave  the  lower  result,  in  13  cases  the 
higher  result. 


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